Preheat the oven to 350 degrees F and line a small loaf pan with parchment paper.
In a microwave-safe bowl, melt the butter and semisweet chocolate chips in 20-second bursts, stirring until smooth.
Stir the sugar into the melted chocolate mixture until fully combined.
Add the egg and vanilla extract, then mix until the batter looks glossy.
Sprinkle in the flour, cocoa powder, and salt, and stir until no dry streaks remain.
Pour the batter into the prepared pan and smooth the top.
Bake for 20 to 22 minutes until the center is just set.
While the brownies cool slightly, melt the white chocolate chips in the microwave in short bursts.
Drizzle the melted white chocolate over the brownies in concentric circles, then drag a toothpick from the center outward to form a spiderweb pattern.
Let the brownies cool completely so the web sets before slicing.